FROMLIST: selftests/mm: include strings.h for ffsl

Got a compilation error on Android for ffsl after 91b80cc5
("selftests: mm: fix map_hugetlb failure on 64K page size systems")
included vm_util.h.

Bug: 331795883
Link: https://lore.kernel.org/linux-kselftest/20240329185814.16304-1-edliaw@google.com/T/#u


Fixes: 6f6a841f ("selftest/vm: add helpers to detect PAGE_SIZE and PAGE_SHIFT")
Change-Id: I71212d2c83c82f0e5293dbfd18055c8f0d3bf649
Signed-off-by: Edward Liaw <edliaw@google.com>